Internet Engineering Task Force (IETF) E. Osborne
Request for Comments: 7324 July 2014
Updates: 6378
Category: Standards Track
ISSN: 2070-1721
Updates to MPLS Transport Profile Linear Protection
Abstract
This document contains a number of updates to the Protection State
Coordination (PSC) logic defined in RFC 6378, "MPLS Transport Profile
(MPLS-TP) Linear Protection". These updates provide some rules and
recommendations around the use of TLVs in PSC, address some issues
raised in an ITU-T liaison statement, and clarify PSC's behavior in a
case not well explained in RFC 6378.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7324.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Osborne Standards Track [Page 1]RFC 7324 PSC Updates July 2014
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Message Formatting and Error Handling . . . . . . . . . . . . 3
2.1. PSC TLV Format . . . . . . . . . . . . . . . . . . . . . 3
2.2. Error Handling . . . . . . . . . . . . . . . . . . . . . 4
2.2.1. Malformed Messages . . . . . . . . . . . . . . . . . 4
2.2.2. Well-Formed but Unknown or Unexpected TLV . . . . . . 4
3. Incorrect Local Status after Failure . . . . . . . . . . . . 5
4. Handling a Capabilities Mismatch . . . . . . . . . . . . . . 5
4.1. Protection Type Mismatch . . . . . . . . . . . . . . . . 5
4.2. R Mismatch . . . . . . . . . . . . . . . . . . . . . . . 6
4.3. Unsupported Modes . . . . . . . . . . . . . . . . . . . . 6
5. Reversion Deadlock Due to a Race Condition . . . . . . . . . 7
6. Clarifying PSC's Behavior in the Face of Multiple Inputs . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 10
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
10.1. Normative References . . . . . . . . . . . . . . . . . . 10
10.2. Informative References . . . . . . . . . . . . . . . . . 10
1. Introduction
This document contains a number of updates to PSC [RFC6378]. One
provides some rules and recommendations around the use of TLVs in
PSC. Three of the updates address issues #2, #7, and #8 as
identified in the ITU's liaison statement "Recommendation ITU-T
G.8131/Y.1382 revision - Linear protection switching for MPLS-TP
networks" [LIAISON]. Another clears up a behavior that was not well
explained in RFC 6378. These updates are not changes to the
protocol's packet format or to PSC's design; they are corrections and
clarifications to specific aspects of the protocol's procedures.
This document does not introduce backward compatibility issues with
implementations of RFC 6378.
It should be noted that [RFC7271] contains protocol mechanisms for an
alternate mode of operating MPLS-TP PSC. Those modes are built on
the message structures and procedures of [RFC6378], and so, while
this document does not update [RFC7271], it has an impact on that
work through its update to [RFC6378].
This document assumes familiarity with RFC 6378 and its terms,
conventions, and acronyms. Any term used in this document but not
defined herein can be found in RFC 6378. In particular, this
document shares the acronyms defined in Section 2.1 of RFC 6378.
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1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. Message Formatting and Error Handling
This section covers message formatting as well as some recommended
error checking.
2.1. PSC TLV Format
[RFC6378] provides the capability to carry TLVs in the PSC messages.
All fields are encoded in network byte order. Each TLV contains
three fields, as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type field (T):
A two-octet field that encodes a type value. The type values are
recorded in the IANA registry "MPLS PSC TLV Registry".
Length field (L):
A two-octet field that encodes the length in octets of the Value
field. The value of this field MUST be a multiple of 4.
Value field (V):
The payload of the TLV. The length of this field (which is the value
of the Length field) MUST be a multiple of 4 octets, and so this
field may contain explicit padding. The length of each single TLV is
the sum of the lengths of its three fields: the length of the value
field + 4. The overall TLV Length field in the PSC message contains
the total length of all TLVs in octets.
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2.2. Error Handling
It is recommended to implement error and bounds checking to ensure
that received messages, if improperly formatted, are handled in such
a way to minimize the impact of this formatting on the behavior of
the network and its devices. This section covers two such areas --
malformed messages and well-formed but unexpected TLVs.
This text is not intended to limit the error or bounds checking a
device performs. The recommendations herein should be taken as a
starting point.
2.2.1. Malformed Messages
An implementation SHOULD:
o Ensure any fields prior to TLV Length are consistent with RFC
6378, particularly Section 4.2 of that document.
o Ensure the overall length of the message matches the value in the
TLV Length + 12.
o Check that the sum of the lengths of all TLVs matches the value in
the TLV Length.
If an implementation receives a message that fails any malformed
message checks, it MUST drop the message and SHOULD alert the
operator to the malformed message. The method(s) used to alert the
operator are outside the scope of this document but may include
things like syslog or console messages.
2.2.2. Well-Formed but Unknown or Unexpected TLV
If a message is deemed to be properly formed, an implementation
SHOULD check all TLVs to ensure that it knows what to do with them.
A well-formed but unknown or unexpected TLV value MUST be ignored,
and the rest of the message processed as if the ignored TLV did not
exist. An implementation detecting a malformed TLV SHOULD alert the
operator as described in Section 2.2.1.
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3. Incorrect Local Status after Failure
Issue #2 in the liaison statement identifies a case where a strict
reading of RFC 6378 leaves a node reporting an inaccurate status:
A node can end up sending incorrect status -- NR(0,1) -- despite the
failure of the protection LSP (P-LSP). This is clearly not correct,
as a node should not be sending NR if it has a local failure. To
address this issue, the fourth bullet in Section 4.3.3.3 of RFC 6378
is replaced with the following three bullets:
o If the current state is due to a local or remote Manual Switch, a
local Signal Fail indication on the protection path SHALL cause
the LER to enter local Unavailable state and begin transmission of
an SF(0,0) message.
o If the LER is in local Protecting Administrative state due to a
local Forced Switch, a local Signal Fail indication on the
protection path SHALL be ignored.
o If the LER is in remote Protecting Administrative state due to a
remote Forced Switch, a local Signal Fail indication on the
protection path SHALL cause the LER to remain in remote Protecting
administrative state and transmit an SF(0,1) message.
4. Handling a Capabilities Mismatch
PSC has no explicit facility to negotiate any properties of the
protection domain. It does, however, have the ability to signal two
properties of that domain, via the Protection Type (PT) and Revertive
(R) bits. RFC 6378 specifies that if these bits do not match an
operator "SHALL [be notified]" (PT, Section 4.2.3) or "SHOULD be
notified" (R, Section 4.2.4). However, there is no text that
specifies the behavior of the end nodes of a protection domain in
case of a mismatch. This section provides that text, as requested by
issue #7 in the liaison statement.
4.1. Protection Type Mismatch
The behavior of the protection domain depends on the exact Protection
Type (PT) mismatch. Section 4.2.3 of RFC 6378 specifies three
protection types -- bidirectional switching using a permanent bridge,
bidirectional switching using a selector bridge, and unidirectional
switching using a permanent bridge. They are abbreviated here as BP,
BS, and UP.
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There are three possible mismatches: {BP, UP}, {BP, BS}, and
{UP, BS}. The priority is:
UP > BS > BP
In other words:
o If the PT mismatch is {BP, UP}, the node transmitting BP MUST
switch to UP mode if it is supported.
o If the PT mismatch is {BP, BS}, the node transmitting BP MUST
switch to BS mode if it is supported.
o If the PT mismatch is {UP, BS}, the node transmitting BS MUST
switch to UP mode if it is supported.
If a node does not support a mode to which it is required to switch,
then that node MUST behave as in Section 4.3.
4.2. R Mismatch
The R bit indicates whether the protection domain is in revertive or
non-revertive behavior. If the R bits do not match, the node
indicating non-revertive MUST switch to Revertive if it is supported.
If it is not supported, a node must behave as in Section 4.3.
4.3. Unsupported Modes
An implementation may not support all three PT modes and/or both R
modes, and thus a pair of nodes may be unable to converge on a common
mode. This creates a permanent mismatch, resolvable only by operator
intervention. An implementation SHOULD alert the operator to an
irreconcilable mismatch.
It is desirable to allow the protection domain to function in a non-
failure mode even if there is a mismatch, as the mismatches of PT or
R have to do with how nodes recover from a failure. An
implementation SHOULD allow traffic to be sent on the Working LSP as
long as there is no failure (e.g., NR state) regardless of any PT or
R mismatch.
If there is a trigger that would cause the protection LSP to be used,
such as SF or MS, a node MUST NOT use the protection LSP to carry
traffic.
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5. Reversion Deadlock Due to a Race Condition
Issue #8 in the liaison statement identifies a deadlock case where
each node can end up sending NR(0,1) when it should instead be in the
process of recovering from the failure (i.e., entering into WTR or
DNR, as appropriate for the protection domain). The root of the
issue is that a pair of nodes can simultaneously enter WTR state,
receive an out-of-date SF-W indication, transition into a remotely
triggered WTR, and remain in remotely triggered WTR waiting for the
other end to trigger a change in status.
In the case identified in issue #8, each node can end up sending
NR(0,1), which is an indication that the transmitting node has no
local failure, but is instead reacting to the remote SF-W. If a node
that receives NR(0,1) is in fact not indicating a local error, the
correct behavior for the receiving node is to take the received
NR(0,1) as an indication that there is no error in the protection
domain, and recovery procedures (WTR or DNR) should begin.
This is addressed by adding the following text as the penultimate
bullet in Section 4.3.3.4 of RFC 6378:
o If a node is in Protecting Failure state due to a remote SF-W and
receives NR(0,1), this SHALL cause the node to begin recovery
procedures. If the LER is configured for revertive behavior, it
enters into Wait-to-Restore state, starts the WTR timer, and
begins transmitting WTR(0,1). If the LER is configured for non-
revertive behavior, it enters into Do-Not-Revert state and begins
transmitting a DNR(0,1) message.
Additionally, the penultimate bullet in Section 4.3.3.3 is changed
from
o A remote NR(0,0) message SHALL be ignored if in local Protecting
administrative state.
to
o A remote No Request message SHALL be ignored if in local
Protecting administrative state.
This indicates that a remote NR triggers the same behavior regardless
of the value of FPath and Path. This change does not directly
address issue #8, but it fixes a similar issue -- if a node receives
NR while in Remote administrative state, the value of FPath and Path
have no bearing on the node's reaction to this NR.
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6. Clarifying PSC's Behavior in the Face of Multiple Inputs
RFC 6378 describes the PSC state machine. Figure 1 in Section 3 of
RFC 6378 shows two inputs into the PSC Control logic -- Local Request
logic and Remote PSC Request. When there is only one input into the
PSC Control logic -- a local request or a remote request but not both
-- the PSC Control logic decides what that input signifies and then
takes one or more actions, as necessary. This is what the PSC State
Machine in Section 4.3 of RFC 6378 describes.
RFC 6378 does not sufficiently describe the behavior in the face of
multiple inputs into the PSC Control Logic (one Local Request and one
Remote Request). This section clarifies the expected behavior.
There are two cases to think about when considering dual inputs into
the PSC Control logic. The first is when the same request is
presented from both local and remote sources. One example of this
case is a Forced Switch (FS) configured on both ends of an LSP. This
will result in the PSC Control logic receiving both a local FS and
remove FS. For convenience, this scenario is written as [L(FS),
R(FS)] -- that is, Local(Forced Switch) and Remote(Forced Switch).
The second case, which is handled in exactly the same way as the
first, is when the two inputs into the PSC Control logic describe
different events. There are a number of variations on this case.
One example is when there is a Lockout of Protection from the Local
request logic and a Signal Fail on the Working path from the Remote
PSC Request. This is shortened to [L(LO), R(SF-W)].
In both cases, the question is not how the PSC Control logic decides
which of these is the one it acts upon. Section 4.3.2 of RFC 6378
lists the priority order and prioritizes the local input over the
remote input in case both inputs are of the same priority. So, in
the first example it is the local SF that drives the PSC Control
logic, and in the second example it is the local Lockout that drives
the PSC Control logic.
The point that this section clears up is around what happens when the
highest-priority input goes away. Consider the first case.
Initially, the PSC Control logic has [L(FS), R(FS)], and L(FS) is
driving PSC's behavior. When L(FS) is removed, but R(FS) remains,
what does PSC do? A strict reading of the Finite State Machine (FSM)
would suggest that PSC transition from PA:F:L into N, and at some
future time (perhaps after the remote request refreshes), PSC would
transition from N to PA:F:R. This is an unreasonable behavior, as
there is no sensible justification for a node behaving as if things
were normal (i.e., N state) when it is clear that they are not.
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RFC 7324 PSC Updates July 2014
The second case is similar. If a node starts with [L(LO), R(SF-W)]
and the local lockout is removed, a strict reading of the state
machine would suggest that the node transition from UA:LO:L to N, and
then at some future time presumably notice the R(SF-W) and transition
from N to PF:W:R. As with the first case, this is clearly not a
useful behavior.
In both cases, the request that was driving PSC's behavior was
removed. What should happen is that the PSC Control logic should,
upon removal of an input, immediately reevaluate all other inputs to
decide on the next course of action. This requires an implementation
to store the most recent local and remote inputs regardless of their
eventual use as triggers for the PSC Control Logic.
There is also a third case. Consider a node with [L(FS), R(LO)]. At
some point in time, the remote node replaces its Lockout request with
a Signal Fail on Working, so that the inputs into the PSC Control
logic on the receiving node go to [L(FS), R(SF-W)]. Similar to the
first two cases, the node should immediately reevaluate both its
local and remote inputs to determine the highest priority among them
and act on that input accordingly. That is in fact what happens, as
defined in Section 4.3.3 of RFC 6378:
When a LER is in a remote state, i.e., state transition in
reaction to a PSC message received from the far-end LER, and
receives a new PSC message from the far-end LER that indicates a
contradictory state, e.g., in remote Unavailable state receiving a
remote FS(1,1) message, then the PSC Control logic SHALL
reevaluate all inputs (both the local input and the remote
message) as if the LER is in the Normal state.
This section extends that paragraph to handle the first two cases.
The essence of the quoted paragraph is that when faced with multiple
inputs, PSC must reevaluate any changes as if it were in Normal
state. So, the quoted paragraph is replaced with the following text:
The PSC Control logic may simultaneously have Local and Remote
requests, and the highest priority of these requests ultimately
drives the behavior of the PSC Control logic. When this highest-
priority request is removed or is replaced with another input,
then the PSC Control logic SHALL immediately reevaluate all inputs
(both the local input and the remote message), transitioning into
a new state only upon reevaluation of all inputs.
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7. Security Considerations
These changes and clarifications raise no new security concerns. RFC
6941 [RFC6941] provides the baseline security discussion for MPLS-TP,
and PSC (as described in both RFC 6378 and this document) falls under
that umbrella. Additionally, Section 2.2 clarifies how to react to
malformed or unexpected messages.
8. IANA Considerations
IANA has marked the value 0 in the "MPLS PSC TLV Registry" as
"Reserved, not to be allocated" and updated the references to show
[RFC6378] and this document (RFC 7324). Note that this document
provides documentation of an action already taken by IANA but not
recorded in RFC 6378.
9. Acknowledgements
The author of this document thanks Taesik Cheung, Alessandro
D'Alessandro, Annamaria Fulignoli, Sagar Soni, George Swallow, and
Yaacov Weingarten for their contributions and review, and Adrian
Farrel for the text of Section 2.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC6378] Weingarten, Y., Bryant, S., Osborne, E., Sprecher, N., and
A. Fulignoli, "MPLS Transport Profile (MPLS-TP) Linear
Protection", RFC 6378, October 2011.
10.2. Informative References
[LIAISON] ITU-T SG15, "Liaison Statement: Recommendation ITU-T
G.8131/Y.1382 revision - Linear protection switching for
MPLS-TP networks", <https://datatracker.ietf.org/
liaison/1205/>.
[RFC6941] Fang, L., Niven-Jenkins, B., Mansfield, S., and R.
Graveman, "MPLS Transport Profile (MPLS-TP) Security
Framework", RFC 6941, April 2013.
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[RFC7271] Ryoo, J., Gray, E., van Helvoort, H., D'Alessandro, A.,
Cheung, T., and E. Osborne, "MPLS Transport Profile (MPLS-
TP) Linear Protection to Match the Operational
Expectations of Synchronous Digital Hierarchy, Optical
Transport Network, and Ethernet Transport Network
Operators", RFC 7271, June 2014.
Author's Address
Eric Osborne
EMail: eric.osborne@notcom.com
Osborne Standards Track [Page 11]
